Commercial and military electronics products are often required to provide EMI and RFI (collectively referred to hereinafter as EMI) shielding around control and display devices. Additionally, electronic components mounted within systems, such as computer systems and signal processing systems must also be shielded from EMI in order to prevent adverse influence on the performance of electronic circuits within the systems. One means of controlling EMI is through the process of shielding, which seeks to confine radiated energy to the bounds of a specific volume or prevent radiated energy from reaching a specific volume. A means for suppressing EMI on printed circuit boards (PCBs) is to provide only one common ground, preferably enclosing or encircling the electronic components. This structure effectively creates a "Faraday Cage" to isolate electronic components on the PCB.
EMI gaskets are well known and are commonly used at the seams and joints of a system, around switches, displays, connectors and around electronic components or groups of components on PCBs. A gasket's shielding effectiveness depends on the material properties of the gasket. Good conductors should be used for electric field shields to obtain high reflection loss and magnetic materials should be used for magnetic field shields to obtain high absorption loss. Shield materials should also be selected for electrochemical corrosion, strength considerations, and conformability.
Many prior art shielding gaskets are formed of a compressible material loaded or wrapped with a conductive material or materials, or a conductive mesh to absorb or reflect EMI. However, these shields are often flimsy, making them difficult to work with and install. Accordingly, there is a need for an EMI gasket having enhanced tensile strength and stiffness properties, while retaining the ability to conform to and around irregular surfaces.
U.S. Pat. No. 5,070,216 to Thornton discloses an EMI shielding gasket for preventing discontinuities in electronic equipment between a cabinet frame and a cabinet door comprising a resilient plastic substrate having an attachment portion and a contact portion, and a metallic conductive coating on an outer surface of the plastic substrate for providing electrical continuity. This gasket has some weaknesses since microwave shielding is determined not only by the DC resistance, but by the degree of conformability of the conductive material, since any resultant gaps or slots will increase radiative emissions/susceptibility and thereby reduce the shielding effectiveness. Further, the gasket is not completely malleable or conformable. Thus, in order to conform to irregular or uneven surfaces, the gasket requires the use of finger like protrusions formed on the bearing portion of the material.
It would be desirable to provide an electrically conductive gasket capable of shielding components from EMI which is semi-rigid so that the gasket is easy to handle and install, yet is highly electrically conductive and provides improved conformability.